Some conventional cooling systems on aircraft utilize air cycle cooling. Hot, pressurized air from the engine is cooled and compressed and used to cool avionics systems and environmental systems, such as the cabin and flight deck. Advancements in composite materials have introduced light yet strong composite components to replace heavier metal components on aircraft. For example, aircraft wings can contain multiple composite components to form a largely composite wing. Composite components do have certain drawbacks, however. Some composite components cannot withstand the high temperatures of the pressurized air bled from the engine that is used for cooling. Thus, using conventional air cycle cooling alone can be unsuitable in some aircraft constructed with composite components. In these cases, alternate cooling systems must be used.
One such alternate cooling system is conventional vapor cycle cooling. Conventional vapor cycle cooling utilizes hydrofluorocarbon refrigerants, such as R-134a. Refrigerant vapor cycle systems offer good performance relative to system weight. Minimizing the weight of aircraft systems increases efficiency of the aircraft and reduces fuel consumption. However, due to environmental concerns and potential regulations, reducing or eliminating the use of R-134a and similar refrigerants is desirable. Other refrigerants, such as carbon dioxide, have lower global warming potential. However, conventional carbon dioxide cooling systems are too heavy to be used in aircraft without incurring a substantial negative effect to efficiency and fuel consumption.